Loop cut (ctrl+R) and space cuts in the same operation
Use the Bevel operator after loop cutting.

Make a single loop cut were you want the symmetrically spaced loops to happen, then bevel with zero segments.

This will virtually replace the single centered loop with two evenly spaced loops. The advantage is it works well in non axis aligned faces, uneven loop widths and allows adding more loops.



Also see related How to slide a vertex group to make new edge loop? and How to outset from edges/vertices?